1. Field of the Invention
The present invention relates to a transmission power control method, and more particularly, to a transmission power control method for a communication system capable of considering path losses of all outgoing channels from a transmitter to receivers, to negotiate a power back-off level which avoids the hidden-node problem, so as to effectively increase power efficiency and data rate of the communication system.
2. Description of the Prior Art
Transmission power back-off technology has been proposed to either increase data rate or reduce power consumption or electromagnetic radiation. However, determining the transmission power back-off level has been a challenge in communication systems that could suffer from the hidden-node problem.
Specifically, since the signal to noise ratio (SNR) dynamic range of an Analog-to-Digital (A/D) converter at a receiver is limited, the transmission power spectral density (PSD) adjustment, the aggregate transmission power adjustment or the gain scaling adjustment will generate better SNR at receiving side at some parts of subcarriers. Accordingly, receiver's throughput will be increased.
For example, please refer to FIG. 1A to FIG. 1B. FIG. 1A is a schematic diagram of a non-flat transmit PSD mask of a powerline communication (PLC) system, wherein PSD masks corresponding to different frequency bands are −55, −85 and −120 dBm/Hz, respectively. FIG. 1B is a schematic diagram of SNR of signals received by a receiver of the PLC system when utilizing the non-flat transmit PSD mask shown in FIG. 1A. As shown in FIG. 1A, in order to comply with the regulation of a country, the transmit PSD mask of the PLC system may not be flat, and the PSD mask for some active subcarriers could be lower than that for the other active subcarriers, e.g. the PSD mask of a high frequency band is lower than the PSD mask of a low frequency band. In certain cases without transmit PSD adjustment as shown in FIG. 1A, an analog automatic gain control (AGC) setting on the receiver can not drive the channel noise above the quantization noise level of an A/D converter for all frequency tones due to limited dynamic range of the converter, and thus those subcarriers with lower reference PSD have lower SNR as shown in FIG. 1B (lower than 25 dB). In other words, since signals in the low frequency band have high transmission power and thus the AGC can only provide a low gain to prevent saturation of the A/D converter, signals in the high frequency band with low transmission power can not be amplified by the AGC with a high gain and thus have low SNR.
On the other hand, please refer to FIG. 1C and FIG. 1D. FIG. 1C is a schematic diagram of a non-flat transmit PSD mask of the PLC system applied with low-band transmission power back-off, wherein PSD masks corresponding to different frequency bands are −65, −85 and −120 dBm/Hz, respectively. FIG. 1D is a schematic diagram of SNR of signals received by the receiver of the PLC system when utilizing the non-flat transmit PSD mask shown in FIG. 1C. As shown in FIG. 1C, if low-band transmission power back-off is applied (10 dBm lower), SNRs of those subcarriers with lower reference PSD masks can be improved significantly (10 dB higher) In other words, if transmission power of signals in the low frequency band is reduced, the AGC can provide higher gain without saturation of the A/D converter, and thus signals in the high frequency band can have higher SNR.
Besides, transmission power back-off has also been suggested to reduce power consumption or electromagnetic radiation. The document IEC CISPR 22 Amd.1 CIS/I/301/CD defines the maximum transmit PSD as a function of the differential mode insertion loss between the equipment under test (EUT) and the auxiliary equipment (AE) for PLC devices. The maximum transmit PSD is −55, −63, and −73 dBm/Hz for channels of insertion losses of >=40, 30, and 20 dB, respectively. It was verified that these PSD limitations will not have any negative impact for well designed PLC devices with a high dynamic range.
As can be seen from the above, the transmitter needs to know a power back-off level that maximizes the benefit of transmission power back-off. However, careless transmission power back-off may result in the hidden-node problem that an on-going packet may be interfered by some distant node which cannot hear the signal from the packet transmitter since transmission power of the signal from the packet transmitter is reduced too much for the distant node to hear due to path loss.
Although the channel estimation procedure for a point-to-point communication, e.g. Ethernet or Digital subscriber line (DSL), in the prior art is effective to provide the characteristics of the communication channel between link partners, each channel is composed of two unidirectional links. During the conventional channel estimation procedure, the transmitter of a link sends sound packets to the receiver; the receiver estimates the channel characteristics and feed the information back to the transmitter; the transmitter utilizes the channel characteristics to optimize the transmission efficiency. The most important channel characteristic probed by this channel estimation procedure is the tone maps, which defines the bit-loading of each tone over an AC cycle. However, such method is not enough to prevent the hidden-node problem since this method only collects information between link partners without considering other listeners on the medium.
Moreover, the PLC system has unique channel characteristics and access scheme, which complicates the transmission power control (TPC) scheme for the PLC system. First, rather than the simple point-to-point connection such as Ethernet, a powerline has complex channel response due to branches or structures similar to bridge taps. Second, powerline is a shared medium for multiple stations and is subject to the hidden-node problem. Third, the channel characteristics change with the AC cycle, i.e. loading and circuit structure change over AC cycles. The bit allocation over the bandwidth of the cyclostationary channel needs complicated tone map management.
Since PLC has the difficulties above, the conventional TPC approaches are not capable for PLC systems. The schemes based on cable length estimation can not work for complex powerline topology and provide no noise information. The schemes purely based on the receiver's measurement will introduce new hidden nodes. Other approaches, such that considering both channel and crosstalk information or per-tone signal and noise power, do not consider the overall channel characteristics and are subject to hidden-node problem. Thus, there is a need to improve over those prior arts.